Note: Descriptions are shown in the official language in which they were submitted.
CA 02334612 2001-02-08
The present invention relates to an automatic
plant, for cutting or welding using a jet of gas,
comprising a line for supplying a torch with pilot gas
and a line for supplying the torch with a cutting or
welding gas, which plant comprises, in addition,
control means enabling the pressure of the cutting or
welding gas to be controlled according to the pressure
of the pilot gas.
Plasma-arc cutting of metal, such as steel
sheets, using a torch delivering a plasma jet is a very
widespread process from the industrial standpoint.
Usually, a cutting operation, for example
involving a steel sheet, breaks down into two distinct
phases, namely an ignition phase and an actual cutting
phase.
During the ignition phase, an ignition gas,
otherwise called the pilot gas, is used to strike, i.e.
ignite the electric arc in the torch.
More specifically, the pilot gas is ionized
within the torch and an electric arc, called the pilot
arc, forms within said torch.
Next, during the cutting phase, the torch is
supplied with a main gas, otherwise called the cutting
gas, which in general has a flow rate of about 10 to
500 1/min, which cutting gas is ionized in the electric
arc which forms between the plasma torch electrode,
acting as cathode, and the workpiece to be cut, acting
as anode.
Depending on the circumstances, the pilot gas
may be identical to or different than the cutting gas,
for example, argon can be used as pilot gas and oxygen,
nitrogen or a gas mixture, such as an argon/hydrogen
mixture, as cutting gas.
However, in all cases, the ignition phase and
the cutting or welding phase are executed one after the
other and in that order.
In general, the flow rates of the pilot gas and
of the cutting gas are set at the desired levels by
CA 02334612 2001-02-08
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adjustment of the pressure of these gases,
respectively.
Moreover, the pilot-gas circuit and the
cutting-gas circuit are usually independent of each
other and the operation of the cutting torch is
increasingly often controlled by automatic control
means, especially control means with a microprocessor
card running one or more computer programs.
However, for some types of gas, such as oxygen
or hydrogen, the use of electrically-controlled
pressure-reducing devices is not possible or
recommended during the cutting phase, for obvious
safety reasons.
Consequently, for this type of gas, the
pressure-reducing devices or the pressure-reducing
valve that can be used are usually of the manual
adjustment type or, depending on the circumstances,
pressure-reducing valves remotely controlled by an
inert gas such as hydrogen or argon, which makes it
possible to alleviate the aforementioned safety problem
but requires the presence of an additional source of
pilot gas.
Conversely, the phase of igniting the torch is
usually carried out with an inert gas with a high
ionizing power, such as argon, with which the use of an
electrically-controlled pressure-reducing valve does
not, by contrast, pose any safety problem.
However, there is a growing demand for devices
for simultaneously and automatically adjusting the
pressure of the pilot gas and of the cutting gas, for
example from the same operator console or via parameter
tables included in computing systems, for example
digital control systems.
This is because, given that the adjustments of
the pilot-gas and cutting-gas pressures depend, in
particular, on the metal sheets to be cut, that is on
the type of material, on its thickness, on its
geometric shape, etc., it is usually difficult for an
operator to find, quickly and with certainty, the
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appropriate parameters for adjusting the pressure of
these two gases.
In other words, the problem which is posed is
that of being able to automatically adjust the pressure
of the cutting gas and the pressure of the ignition gas
in complete safety, whatever type of gas is used and
without using an external source of pilot gas from the
pressure-reducing valves fitted in the lines or
circuits conveying, on the one hand, the pilot gas and,
on the other, the cutting gas.
The solution provided by the present invention
is based on control of the pressure-reducing valve
fitted in the cutting-gas circuit, directly on the
basis of the pressure of the pilot gas flowing in the
pilot-gas circuit.
In other words, the present invention relates
to an automatic plant for cutting or welding by a
cutting or welding gas jet, especially by plasma jet,
comprising at least:
- a torch for cutting by gas jet, preferably a
plasma torch;
- a first line for supplying said torch with a
pilot gas;
a second line for supplying said torch with a
cutting or welding gas;
- control means;
- first means for adjusting the pressure of the
pilot gas flowing in the first line, said first
adjusting means being fitted in the first line and
being controlled by the control means;
- second means for adjusting the pressure of the
cutting or welding gas flowing in the second line; and
- a pneumatic control line pneumatically
connecting the first line to the second adjustment
means in order to enable the pressure of the cutting or
welding gas in the second line to be adjusted on the
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basis of the pressure of the pilot gas in the first
line by acting on said second adjustment means.
Depending on the case, the plant according to
the invention may comprise one or more of the following
characteristics:
- the torch is a plasma jet cutting torch;
- the first adjustment means are electrically
controlled, preferably first adjustment means are
chosen from electrically-controlled pressure-reducing
valves;
- the control means are chosen from numerical
control systems;
- the second adjustment means are chosen from
pneumatically-controlled pressure-reducing valves;
- at least one solenoid valve is fitted in the
first line and/or the second line, preferably at least
one solenoid valve is fitted in each of said first and
second lines;
- at least one solenoid valve is controlled by
the control means;
- the control line is connected to the first
line, downstream of said first adjustment means.
According to another aspect, the present
invention also relates to a process for cutting or
welding by a cutting or welding gas jet, in particular
plasma jet, capable of being implemented by an
automatic plant, such as the aforementioned plant,
comprising means for adjusting the pressure of the
cutting gas flowing in a line for supplying a cutting
torch with cutting or welding gas, in which the means
for pneumatically adjusting the cutting or welding gas
are controlled on the basis of the pressure of the
pilot gas.
The present invention will now be described in
more detail using the appended figures, which are given
by way of non-limiting illustration.
CA 02334612 2001-02-08
Figures 1 and 2 show, schematically and
respectively, the ignition phase (Fig. 1) and the
succeeding cutting phase (Fig. 2) during an operation
of cutting, with a plasma arc, a workpiece PC to be
5 cut, for example a steel sheet.
In Figure I, it can be seen that, during the
phase of igniting the pilot arc, a transfer of
electrons and the appearance of one or more pilot arcs
between the electrode E of the plasma torch T and the
nozzle TU of said plasma torch T take place.
During this phase of igniting the pilot arc,
the plasma torch T is supplied with pilot gas GP, for
example argon, so as to obtain a plasma flux FP
delivered by the torch T in the direction of the
workpiece PC to be cut.
In addition, during this ignition phase, the
torch T is supplied with electric current only by an
auxiliary source SA of electric current, for example a
current at a voltage of 100 V and with an intensity of
15 A, while, during this ignition phase, the main
source SP of electric current does not operate
(U = 0 V, I = 0 A) .
Moreover, Figure 2 shows diagrammatically the
cutting phase following the ignition phase shown in
Figure 1. More specifically, in Fig. 2, during the
cutting phase, the pilot gas is replaced by a cutting
gas which may be of the same type as or of a type
different than the pilot gas, for example oxygen or a
mixture of argon and hydrogen.
In addition, during the cutting phase, it is
found that a stream of electrons appears between the
electrode E and the workpiece PC to be cut until
drilling and cutting along a kerf SC of said workpiece
PC are obtained.
During the cutting phase, the auxiliary source
SA of electric current is disconnected (U - 0 V;
I = 0 A), while the main source SP of electric current
delivers a current having an intensity and a voltage
which can vary according to the thickness of the
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workpiece to be cut, for example, a voltage between 100
and 300 V and an intensity between 15 and 1000 A.
Figure 3 shows diagramatically the supply
system for a plasma torch T supplied with pilot gas and
with cutting gas via two different gas circuits which
are independent of each other.
More specifically, the pilot gas circuit
comprises a pilot-gas supply line L1 connected,
upstream, to a pilot-gas source S1 and, downstream, to
the plasma torch T, and in which line Ll are fitted, on
the one hand, means MRl for manually adjusting the
pressure of the pilot gas and a solenoid valve EV1
controlling the flow of said pilot gas to the plasma
torch T.
Similarly, the cutting-gas circuit comprises a
cutting-gas line L2 connected, upstream, to a cutting-
gas source S2 and, downstream, to the plasma torch T,
and in which line L2 are fitted, on the one hand, means
MR2 for manually adjusting the pressure of the cutting
gas and a solenoid valve EV2 controlling the flow of
cutting gas to the plasma torch T.
This type of system, known from the prior art,
has many disadvantages, especially that of requiring
manual adjustment of the pilot-gas pressure and of the
cutting-gas pressure by direct action by the operator
on the adjustment means MR1 and MR2, respectively.
Figures 4 and 5 show the layout of a system for
supplying a plasma torch T with a pilot gas and with a
cutting gas, respectively, according to the present
invention.
More specifically, Figure 4 shows
diagramatically the operation of the system for
supplying pilot gas and cutting gas during the ignition
phase, whereas Figure 5 shows the operation of the
supply system during the cutting phase.
In Figure 4, it can be seen that, during the
ignition phase, the pilot gas is transported to the
plasma torch T by the line L1, provided that the
solenoid valve EVl operated by the control means MP is
CA 02334612 2001-02-08
in the open position, i.e. the valve allows pilot gas
to pass through to the torch T and, in addition, the
electrically- or pneumatically-controlled solenoid
valve EV2, in the closed position, then prohibits
cutting gas passing through to the torch T.
The pressure of the pilot gas in the pilot-gas
line L1 is adjusted via electrically-controlled
pressure-adjustment means MRl, for example an
electrically-controlled pressure-reducing valve
controlled by control means MP.
In contrast, the pressure of the cutting-gas in
the cutting-gas supply line is adjusted, in this case,
by pneumatic-control pressure-adjustment means MR2,
which are controlled by the pressure of the pilot gas
in the pilot-gas line Ll.
This is because the pilot-gas line Ll has a
bypass or pneumatic control line LP, connected to the
pilot-gas line L1, at a connection point SL located
downstream of the adj ustment means MR1 but upstream of
the solenoid valve EV1.
In this wav, by ac-t i n~ ~~n tr,o "; ~ "~-..-,~
pressure-adjustment means MR1, the control means MP
generate a first pressure P1 of the pilot gas
downstream from said adjustment means MR1, in
particular in the pneumatic control line LP, which
first pressure Pl will act on the cutting-gas pressure
adjustment means MR2 in order to set the desired
cutting-gas pressure level PX when the flow rate of
said cutting gas is zero (EV2 being in the closed
position).
Then, as shown in Figure 5, during the cutting
phase the process is reversed, i.e. the control means
control the solenoid valve EVl which closes,
prohibiting the pilot gas from passing through to the
plasma torch T and, in addition, controlling the
solenoid valve EV2 which opens, allowing the cutting
gas to pass through to the torch T.
In addition, the control means also act on the
pilot-gas pressure-adjustment means MR1 so as to set
CA 02334612 2001-02-08
the pressure of the pilot gas to a second pressure
level P2, which generates a change of pressure within
the pneumatic control line LP, which pressure change
has a knock-on effect on the cutting-gas pressure-
s adjustment means MR2, which are then controlled in
order to adjust the cutting-gas pressure to a desired
cutting-gas pressure and flow rate.
It is therefore understood that the device
according to the present invention makes it possible to
use any type of gas as cutting gas, especially gases
likely to pose safety problems, such as oxygen or
hydrogen, provided that, according to the invention,
the cutting-gas pressure-adjustment means are
pneumatically controlled, which prevents any danger,
provided that the cutting-gas pressure-adjustment means
MR2 is not electrically controlled.
In other words, according to the present
invention, it is possible to control the flow rate and
pressure of two different gases, namely a pilot gas and
a cutting gas, requiring pressure and flow rate
adjustments which are different from one another, with
a single adjustable electric set-point line.
The present invention is applicable to
automat_c cutting machines using a plasma torch or an
oxycutting torch, but also to an automatic machine
using a welding torch or, more generally, to any type
of machine for delivering gas with a high flow rate, in
particular oxygen and/or hydrogen, requiring a sequence
of pressures of different gases on the basis of one and
the same electrical set point origin.
